Grid-Side Converters Control and Design

The power systems, power engineering, and power industry are all changing in rapid manner. There are several contemporary drives for such a change. The exhaustion of fossil fuels, the introduction of renewable sources, the increase in electrical vehicles, distributed generation, distributed accumulation, real-time negotiated energy transactions, and the introduction of electronic controlled loads and sources with both local and remote supervision and control are all calling for fast and substantial changes in generation, transmission, distribution, and use of the electrical energy.

Digital current controllers are used in grid-side inverters, generator-side inverters, electrical drives, and many other applications. Their purpose is to control the electrical current injected into the grid or the electrical current supplied to the windings of an electrical machine. Desired goals include quick and accurate suppression of any errors between the desired current, also called the current reference, and the actual current obtained from the current sensors. Ideally, digital current controllers should provide the means for an error-free tracking of the current reference profiles even in the presence of the input disturbances (i.e., sudden changes of the reference) and the voltage disturbances (sudden changes of the line voltages, back-electromotive forces, and similar). The driving force in current controllers is the voltage supplied to the load, wherein the load could be either the LCL filter in grid-side inverters or the ac machine stator winding in electrical drives. In essence, the current controller uses the current error to calculate the required voltage that would drive detected error back to zero. The controller outputs such voltage as a signal called the voltage reference. It takes the voltage actuator to generate the actual voltage that corresponds to the voltage reference. In other words, the voltage actuator is an amplifier that turns the voltage reference signals into power circuit voltages.

Closed-loop performance of grid-side inverters and electrical drives depend on the closed-loop bandwidth of digital current controllers, which are used as inner control loop in vast number of cases. The current control is organized on the feedback principles. Thus, it is necessary to measure the output currents for the purpose of closing the feedback and also for the purpose of overcurrent protection. The feedback acquisition chain comprises analogue and digital devices and filters. Their purpose is producing the digital word that represents the output current with the least possible delay and error. The digital word resides within RAM of the digital controller, and it is used for the feedback and protection purposes.

This chapter introduces the basics of three-phase digital current controllers. Their practical implementation is digital, and it involves the PWM actuator and the feedback acquisition systems described in the previous chapter. The analysis and design of discrete-time current controllers rely on z-transform, and it takes into consideration all the feedback acquisition, computation, and PWM delays. In order to facilitate the introduction of some basic principles of the current control, this chapter does not take into account the discrete nature of the controller, nor does it consider the transport delays. Instead, the analysis is performed in s-domain.

In this Chapter, the closed loop performance of digital current controllers is improved by introducing advanced scheduling of the control tasks. The Chapter provides the relevant analysis, an insight into the available scheduling options; and it discusses the scheduling schemes that have the potential of reducing the transport delay within digital current controllers. For the most promising scheduling schemes, the analysis provides the design and parameter setting along with an evaluation and comparison to the previous solutions.

Digital current controllers have the crucial impact on performance of grid-side converters and ac drives. The tasks of the current controller include an error-free tracking of the input reference but also the suppression of the voltage disturbance. In ac drives, the voltage disturbances are the back-electromotive forces of ac machines. In grid-side inverters, the voltage disturbances are the line voltages. The voltage disturbances are commonly suppressed by enhancing the controller with an inner active resistance feedback, as described in Sect. 4.​5. In cases where the switching noise and parasitic oscillations introduce sampling errors, conventional sampling is replaced by the oversampling-based error-free feedback acquisition which derives the average of the measured currents over the past switching period. This one-PWM-period feedback averaging is introduced in Sect. 3.​3. The time delay introduced into the feedback path creates difficulties in designing the current controller with the active resistance. In this chapter, the possibility of applying the active resistance feedback in systems with the error-free sampling is introduced and discussed. The analysis is focused on studying the impact of transport delays, introduced by the feedback averaging on the range of the applicable active resistance gains. The internal model principle is applied in order to get the modified current controller where the active resistance gain does not affect the input step response. Disturbance rejection capability is tested analytically, by computer simulation and experimentally.

The grids for transmission and distribution of electric energy have an ever-increasing share of static power converters. They include the source-side converters, the bus converters, and the load-side converters. Typical source-side converters are the inverters that collect the electric energy from the wind power plants or solar power plants, convert the energy into a set of three-phase voltages and currents, and inject the active and reactive power into the three-phase ac grid. The bus power converters are used for connections between the grids of different voltage levels, and they can be either ac/ac, dc/dc, or ac/dc. In a way, the bus converters tend to replace the traditional line-frequency power transformers. The load-side power converters are used as the power interface between the grid and the load. They convert the grid voltages and adjust the load voltages to suit the needs of the electric power application. With the advent of local accumulation and considering the regeneration needs of electrical drives, most load-side converters have to be bidirectional, capable of supplying electric energy into the grid during brief intervals of time. Therefore, the basic functionality of all the grid-side converters is similar. When interfacing the ac grids, the grid-side power converter has to provide the voltages and inject the currents that are in synchronism with the grid voltages. Therefore, it is necessary to provide the means for detecting the frequency and the phase of the grid ac voltages. Most common device in use is the phase-locked loop (PLL), often used in radio circuits.